The cause of asthma has long remained one of the most tantalising of medical mysteries - particularly as the disease is on the increase world-wide.

But scientists have come up with two pieces of research that provide compelling evidence that bacteria are to blame.

Researchers in Finland and US have shown that the disease may be triggered by proteins produced by a common bacterial infection.

And separate work by US researchers has shown that antibiotics can help ease sufferers' symptoms.

The guilty party appears to be a bug called Chlamydia pneumoniae, already known to be a common cause of lung infections.

Blood samples

A team from the National Public Health Institute in Oulu, Finland, tested the theory that asthmatics are much more sensitive than other people to the presence of a protein produced by C. pneumoniae.

It is too early to know whether such an association is genuine

Dr Martyn Partridge

They analysed blood samples taken from asthmatics, people with bronchitis and healthy people.

They found that the blood of asthmatics was much more likely to contain tell-tale signs of an immune system response to C. pneumoniae protein.

Lead researcher Dr Maija Leinonen told New Scientist magazine: "It's the first time that this protein has been shown to be associated with asthma."

Dr Leinonen believes that long-term infections may put the immune system into over-ride, leading to inflammation and asthma attacks.

If the theory is correct, it suggests that antibiotics could help asthma sufferers.

This was exactly what happened when the drugs were given to asthmatics in a trial carried out by researchers from the National Jewish Medical Research Center in Denver.

Daily doses

The scientists gave 55 people with asthma twice-daily doses of an antibiotic called clarithromycin for six weeks.

About half the volunteers were infected with C. pneumoniae or another bacterium called Mycoplasma pneumoniae - and these people showed clear improvements in lung function.

The researchers believe that C. pneumoniae could play a role in as many as half of all cases of asthma in adults.

However, Dr Margaret Hammerschlag, of the State University of New York, is sceptical.

She found that the bacterium was only present in a minority of children with asthma, and in no adult sufferers at all.

She said: "It probably does play a role sometimes. But it ain't the only player."

Dr Martyn Partridge, chief medical advisor
for the National Asthma Campaign, told BBC News Online: "This is certainly an interesting paper however, it is too early to know whether such an association is genuine and therefore it is too early to talk
about treating it with antibiotics.

"It is an area which requires more work and I shall be watching with interest."